_oid should work for OIDs that *don't* correspond to real Things

(User Story, Closed -> Fixed, Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Functions
Summary: Probably never useful for user-level code, but I need this for the App Gallery
Specifically, the App Gallery needs the OIDs of the owner's identity and the app itself, to be able to build the URL of the App.
This actually just involves making _oid more efficient -- don't bother to look up the linked Thing, since all we actually need is the received OID.